1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device operated in a bipolar transistor mode.
2. Description of the Prior Art
A conventional GaAs lateral bipolar transistor is described, for example, in the Proceedings of the 45th Meeting on Japan Society of Applied Physics, Lecture No. 15a-H-9 (1984). In this GaAs lateral bipolar transistor, an n-type emitter region 2 and an n-type collector region 3 are buried in a semi-insulating GaAs substrate 1 at a predetermined distance, as shown in FIG. 1. A p-type region 4 (hole injector) is formed in the GaAs substrate 1 between the emitter region 2 and the collector region 3. An Si.sub.3 N.sub.4 film 5 as a passivation film is formed on the GaAs substrate 1. An emitter electrode 6, a base electrode 7, and a collector electrode 8 are formed through openings 5a to 5c of the Si.sub.3 N.sub.4 film 5, respectively.
In the GaAs lateral bipolar transistor shown in FIG. 1, when a forward bias voltage is applied between the emitter region 2 and the p-type region 4, a virtual base region 9 is formed by holes (majority carriers) injected from the p-type region 4 into the GaAs substrate 1. The virtual base region 9 accelerates injection of electrons from the emitter region 2 into the GaAs substrate 1. Thus, the bipolar transistor operation is enabled so that the injected electrons reach the collector region 3 through the virtual base region 9, as indicated by arrow A.
The conventional GaAs lateral bipolar transistor shown in FIG. 1 has the following drawbacks. First, it is difficult to sufficiently increase a DC current gain h.sub.FE because of a large recombination current caused by recombination at defects present on a surface 1a of the GaAs substrate 1 between the emitter region 2 and the p-type region 4. Secondly, since a lateral structure is adopted, distance between the virtual base region 9 and the collector region 3 is large. For this reason, since an electron recombination rate in a bulk (the GaAs substrate 1) is high, the recombination current also flows in the bulk. Thirdly, transistor characteristics vary widely because of variations in distance between the emitter region 2 and the p-type region 4. Fourthly, if a different GaAs substrate 1 is used, since influence of trap differs, the transistor characteristics may vary widely in accordance with the GaAs substrate used.